1. Field of the Invention
This invention relates to an improved glass inlet system for injecting a sample into a mass spectrometer or the like. More specifically, the invention relates to a vacuum lock for an inlet system with improved performance during heat up.
2. Description of the Prior Art
The introduction or injection of a small sample of vapors of a high molecular weight compound into various scientific apparatus, equipment, analytical instrument, or the like has historically presented a serious problem. In order to volatilize such a sample high temperatures and low pressures are required. Such conditions inherently imply heating with an associated thermal expansion problem as well as sample stability or compatibility considerations. Thus, in designing and building an acceptable sample inlet system, particularly when the system is to be used repeatedly such as when a series of samples are to be injected into a mass spectrometer, one must account for repeated expansion and contraction.
One of the earliest designs for a mass spectrometer inlet system which required high temperature vaporization of the sample was a gallium covered glass frit inlet. Later, a gallium orifice inlet was introduced that eliminated the problem of sample fractionation caused by the frit, but had an upper temperature limit of about 300.degree. C. and retained the property of potential catalytic reaction by contact of the sample with hot gallium. Various all glass inlet systems have also been described in scientific literature. In one system a weighed sample sealed in a capillary tube is introduced into an expansion vessel and then broken with a magnetic plunger. Another all glass heated mass spectrometer inlet which extends the upper operating temperature from about 300.degree. to 450.degree. C. for very small samples (0.1 milligrams) is disclosed in J. Mass Spectrometry and Ion Physics (1968), pages 87-92 and is the subject matter of U.S. Pat. No. 3,594,574. This inlet system includes a valved manifold and a vacuum lock for sealingly connecting a sample container to the manifold. The lock chamber is formed by two concentric tubes joined at one end by a ring seal and polished to an optical flat mating surface at the other end. A vacuum outlet tube is connected to the annular space or central chamber between the concentric tubes such that a sample ampoule tube with optical flat mating surface can be held against this vacuum lock during operation. This optical flat vacuum lock inlet system has proved to be a reliable method for introducing small samples at higher temperatures without loss of vacuum in the mass spectrometer, yet is limited in that the presence of the concentric tubes create dissimilar rates of thermal expansion upon heating. The present invention is viewed as a specific improvement to the glass inlet system of U.S. Pat. No. 3,594,574.